Minimizing radiation damage in nonlinear optical crystals

ABSTRACT

Methods are disclosed for minimizing laser induced damage to nonlinear crystals, such as KTP crystals, involving various means for electrically grounding the crystals in order to diffuse electrical discharges within the crystals caused by the incident laser beam. In certain embodiments, electrically conductive material is deposited onto or into surfaces of the nonlinear crystals and the electrically conductive surfaces are connected to an electrical ground. To minimize electrical discharges on crystal surfaces that are not covered by the grounded electrically conductive material, a vacuum may be created around the nonlinear crystal.

This application is a divisional application of Ser. No. 08/630,305,filed Apr. 10, 1996, now U.S. Pat. No. 5,805,329, issued Sep. 9, 1998.

FIELD OF THE INVENTION

The present invention generally relates to nonlinear optical crystals,and, more specifically, to methods for minimizing the damage tononlinear optical crystals brought about by optical radiation. Thisinvention was made with Government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Nonlinear optical crystals are used extensively throughout the world ina great many optical applications. But for problems associated withradiation damage, their use would be even greater. Currently, the mostcommon use of nonlinear crystals appears to be as second harmonicgenerators (SHGs) and as optical parametric oscillators (OPOs).

Most of the nonlinear crystals now being produced are capable ofhandling relatively high power radiation over a limited period of time.The useful lifetime of these nonlinear crystals is limited because thelarge electric fields associated with most crystal applications cause,over time, electrochromic and photochromic damage to the crystal. Thisprogressive damage can severely limit the average-power use of theseexpensive crystals.

Single crystal KTiOPO₄ (KTP) probably is the most widely used nonlinearoptical material currently available. The excellent crystal stabilityand large nonlinear optical coefficient make KTP crystals the materialof choice for both SHG and OPO applications. Because of the extensiveuse of KTP crystals, the radiation damage suffered by most nonlinearcrystals is documented extensively for KTP crystals.

In the approximately twenty years which have elapsed since thecommercialization of KTP crystals, a great deal of work has been devotedto understanding the origin of this radiation damage and developingeffective methods for mitigating its effects. This radiation damage,also referred to as "photochromic damage" or "gray tracking," previouslyhas been thought to occur when an oxygen-hole pair is generated and theelectron is trapped on a Ti⁴⁺ ion, creating a stable Ti³⁺ ion. Opticalabsorption of Ti³⁺ ions occurs in the green region of the spectrum, andhas been thought to be responsible for the gray tracking problem.

In SHG applications, the converted green light (˜530 nm) was consideredto be absorbed by the Ti³⁺, leading to heating and eventual fracture ofthe KTP crystal. It was postulated that the above-gap energy necessaryfor the creation of an oxygen-hole/electron pair can result from eithersum-frequency mixing of the fundamental and second harmonic frequencies,sum frequency mixing of the Raman-shifted fundamental and secondharmonic frequencies, or direct two-photon absorption of the generatedgreen light.

Electrochromic damage in nonlinear crystals generally occurs whenexposed to electric fields of a few kV/cm for a period of time. After athreshold field has been reached, the crystal breaks down, andgreen/black streaks appear in the material along the direction of theapplied field. These streaked areas in the crystals exhibit absorptionsimilar to the gray-tracked material, suggesting the conclusion that, inKTP crystals, the Ti³⁺ ions were associated with the damage.

When damaged crystals were cut open, an internal examination revealed anexcess of Ti and K on the crystal's surface, and a deficiency of theseelements in the internal regions. This finding led to the conclusionthat Ti and K migrate under the application of a sufficiently strongelectric field, and that the electrochromic damage was most likely acombination of bulk damage caused by this migration and the concomitantcreation of Ti³⁺ ions in the KTP crystal. The present invention teachesthat this is not the damage mechanism.

It is therefore an object of the present invention to provide methodsfor minimizing or preventing electrochromic damage to nonlinearcrystals.

It is another object of the present invention to provide apparatus forallowing the electrical grounding of crystals.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method ofminimizing electrochromic damage in nonlinear crystals comprising thesteps of placing an electrical conductor in close proximity to thenonlinear crystal, and grounding the electrical conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a schematical representation of a nonlinear crystal beingradiated with a laser beam, with representations of void areas withinthe crystal containing entrained nitrogen, along with an enlargementillustrating the electric field created within the crystal and theelectronic combination which results in gray tracking in the crystal.

FIG. 2 is a schematical representation of a rectangular cross-sectionnonlinear crystal with a grounding wire placed in close proximity to thecrystal, and a laser beam being perpendicular to the crystal.

FIG. 3 is a schematical representation of a square cross-sectionnonlinear crystal with a conducting material placed on two faces of thecrystal, the conducting material being grounded and a laser beam beingperpendicular to the crystal.

FIG. 4 is a schematical representation of a square cross-sectionnonlinear crystal with conducting material placed on four faces of thecrystal, a single connection to ground and a laser beam beingperpendicular to the crystal.

FIG. 5 is a schematical representation of a rectangular cross-sectionnonlinear crystal with conducting material placed on four faces of thecrystal, a single connection to ground, and an elliptical laser beambeing perpendicular to the crystal.

DETAILED DESCRIPTION

The present invention provides methods for minimizing electrochromicdamage in nonlinear crystals. Although the general solution tominimizing this very important problem in the industrial application ofthese crystals may seem simple, the route to discovering this solutionhas been most difficult.

The reasons for photochromic damage in nonlinear crystals have eludedresearchers since soon after the introduction of these crystals. Thesenonlinear crystals are known to produce what has been generally termed"pyroelectric luminescence," and are generally classed as pyroelectriccrystals. Under appropriate conditions, electrical discharge may occurin any pyroelectric crystal, a term which includes all nonlinearcrystals.

Reference should now be made to FIG. 1, where nonlinear crystal 11 isschematically illustrated. It now has been shown that currently producednonlinear crystals 11 contain voids 12 which contain entrained nitrogen.The incidence of laser beam 13 onto nonlinear crystal 11 and itsinteraciton with voids 12 produces the situation shown in theenlargement also illustrated in FIG. 1.

The incidence of laser beam 13 produces heating in central portion 11aof nonlinear crystal 11 while laser beam 13 is on, and cooling whenlaser beam 13 is off. Voids 12, as stated, have been shown to containentrained molecular nitrogen, which has a dielectric constant ofapproximately 1 (ε˜1), compared to typical specimens of nonlinearcrystal 11 having dielectric constants in the range of ε˜10 to 50. Therandom discharges observed in nonlinear crystals 11 upon heating orcooling, measured as pyroelectric luminescence, occur at voids 12.

Voids 12 can be viewed, as shown in the enlargement in FIG. 1, asparallel plate capacitor 14 enclosing void 12. Capacitors 14 store thecharge generated by the changing temperature of nonlinear crystal 11.Since nonlinear crystals 11 have a finite conductivity, capacitors 14will gradually discharge over time.

Pyroelectric crystals in general, develop an internal electric field dueto changes in the spontaneous polarization as a function of temperatureaccording to the following: ##EQU1## where i is the current generateddue to the electric field, λ_(T) is the pyroelectric coefficient and βis the heating rate. When nonlinear crystal 11 is heated or cooled, anelectric field, E, is generated within nonlinear crystal 11. Thepyroelectric coefficient of nonlinear crystal 11 is related to thisfield as follows: ##EQU2## where ε is the dielectric constant of themedium. It is thereby seen that a changing temperature acts as thegenerator of net charge developed within nonlinear crystal 11, and theaccompanying field, E, may reach sufficient value to discharge. It isthis buildup of electric field, E, and its subsequent discharge thatproduces current flow across pyroelectric crystals, accompanied byoptical emission.

It has been shown that regardless of the ambient environment ofnonlinear crystal 11 or the particular type of nonlinear crystal 11being investigated, this optical emission is always characterized bydiscrete spectral lines (band heads) in the near ultraviolet and visibleportion of the electromagnetic spectrum and are due to ionization ofmolecular nitrogen entrained within voids 12.

As previously explained, these electrical discharges can occur withinany nonlinear crystal 11 and are the vehicle which produces theelectrochromic damage over time. This thermally-induced damage, whichcan be produced by the rapid local heating caused by laser beam 13, orby any thermal cycling procedure, will be detrimental to the attractiveoptical properties of nonlinear crystals 11.

As an example, potassium titanyl phosphate (KTP) nonlinear crystals 11are used extensively for frequency doubling Nd:YAG lasers shown inFIG. 1. Laser beam 13 is incident at a wavelength 1064 nm, and outputfrom nonlinear crystal 11 at a wavelength of 532 nm. The principallimitation on the application of KTP nonlinear crystals 11 in high powerlaser applications is laser-induced damage known as "gray tracking."Although KTP crystals were developed in 1976, the origin of this graytracking problem has not been known.

The present invention teaches that gray tracking primarily results fromthe heating and subsequent cooling produced in nonlinear crystal 11 bypulsed lasers. These heating and cooling cycles produce the electricaldischarges described above in the low dielectric constant voids 12 whichcontain entrained molecular nitrogen. The ionization process caused bythese electrical discharges produces free electrons, probably fromoxygen atoms, which are captured by Ti⁴⁺ ions of the KTP crystallattice, converting the Ti⁴⁺ ions to Ti³⁺ ions.

The Ti³⁺ ions absorb in the green portion of the electromagneticspectrum, which is also the wavelength of the frequency-doubled light,532 nm. This process quickly leads to catastrophic damage to nonlinearcrystal 11 because green light is being produced and simultaneouslyabsorbed by nonlinear crystal 11.

It is worthy of note that minimal damage to nonlinear crystal 11 occurswith a cw laser beam 13. However, with pulsed laser beams 13catastrophic damage occurs. The reason for this effect is that under cwoperation, there is no change in spontaneous polarization withtemperature, except at the beginning and end of laser excitation. Withcw operation, dynamic equilibrium is quickly attained, and dP_(s) /dtbecomes zero. However, this is not true for pulsed laser operation,where dP_(s) /dt is dependent on the laser pulse rate, causing dP_(s)/dt to be nonzero. For high power applications of nonlinear crystal 11,pulsed laser operation is required.

Currently, the usefulness of KTP nonlinear crystals in frequencydoubling applications is limited by the laser-induced damage problem.The present invention teaches that one way of minimizing thelaser-induced damage is to dissipate the electrical charge that buildsup as a result of laser heating. When this is done efficiently, theradiation damage of gray tracking can be minimized. The general methodtaught by the present invention is electrical grounding of nonlinearcrystal 11 to quickly drain excess electrical charge away from nonlinearcrystal 11.

To accomplish this grounding of nonlinear crystal 11 reference should bedirected toward FIGS. 2-5. In FIG. 2, perhaps the easiest method ofproviding a ground to existing nonlinear crystals 11 is to place anelectrically conductive material 21, such as a copper, aluminum, silveror gold wire, within 1 to 2 mm of nonlinear crystal 11, and to connectthe opposite end to ground 22. This configuration is similar to theoperation of a lightning rod.

Nonlinear crystals 11 are typically in form of a parallelepiped,approximately 3 mm×3 mm×5 mm. FIG. 3 illustrates another groundingmethod appropriate for such a configuration in which electricallyconductive material 31 is applied to two opposite surfaces of nonlinearcrystal 11 which are parallel to laser beam 13. Electrically conductivematerial 31 is connected to ground 22 in any convenient manner.

FIG. 4 illustrates another grounding method in which electricallyconductive material 41 is applied to all four faces of nonlinear crystal11. As in FIG. 3, electrically conductive material 41 is parallel tolaser beam 13. This method brings with it a likelihood of quicklydischarging any electric fields within nonlinear crystal 11.

FIG. 5 illustrates a configuration for nonlinear crystal 11 which wouldimprove the power density in nonlinear crystal 11 when employed infrequency doubling applications. As seen in FIG. 5, laser beam 13 iselliptical, and for this application would have a thickness of only afew hundred micrometers. Similarly, nonlinear crystal 11 is onlyslightly thicker than laser beam 13. Electrically conductive material 53is applied to all four sides of nonlinear crystal 11 and connected toground 22. Additionally, nonlinear crystal 11 can be mounted onto anelectrically conductive surface, such as a copper block, which is alsoconnected to ground 22.

The configuration of FIG. 5 is important to frequency doublingapplications because in those applications maximizing power density isimportant. However, with increased power density, laser-induced heatingwill also be proportionally increased. For this reason, it is importantthat electrical charges be removed quickly. To accomplish rapid chargeremoval, it is necessary to have grounded electrically conductivematerial 53 as close to the region of nonlinear crystal 11 in whichcharges build up as possible. A thin elliptical laser beam 13 and a thinparallelepiped nonlinear crystal 11 allow close proximity between thecharges and electrically conductive material 53, enhancing the rate ofremoval of rapidly induced electrical charge, and minimizing radiationdamage to nonlinear crystal 11.

In the embodiments shown in FIGS. 3, 4 and 5, it may be advantageous toalso deposit electrically conductive material 31 (FIG. 3), 41 (FIG. 4)and 53 (FIG. 5) on the portion of the surfaces of nonlinear crystal 11which are perpendicular to laser beam 13 and which is not in the path oflaser beam 13. This provides grounding to at least a portion of all sixsurfaces of nonlinear crystal 11 to increase the rate of chargedissipation.

Electrically conductive material 31 (FIG. 3), 41 (FIG. 4) and 53 (FIG.5) may comprise any suitable electrically conductive material,including, but not limited to copper, aluminum, silver and gold. Thesematerials may be applied to nonlinear crystal 11 in any appropriatemanner. For instance, sheets of electrically conductive material 31(FIG. 3), 41 (FIG. 4) and 53 (FIG. 5) could be clamped onto nonlinearcrystals 11. Alternatively, electrically conductive material 31 (FIG.3), 41 (FIG. 4) and 53 (FIG. 5) could be deposited onto nonlinearcrystal 11 by chemical or physical deposition techniques. Additionally,electrically conductive material 31 (FIG. 3), 41 (FIG. 4) and 53 (FIG.5) can be diffused into nonlinear crystal 11 so that it is in closerproximity to the region affected by laser beam 13. This diffusion alsowould improve the rate at which electrical charge could be dissipated.

Connecting electrically conductive material 31 (FIG. 3), 41 (FIG. 4) and53 (FIG. 5) to ground 22 can be made in any appropriate manner. Forexample, one surface of nonlinear crystal 11 could simply be in physicalcontact with a grounded electrically conductive block. Alternatively, awire compatible with electrically conductive material 31 (FIG. 3), 41(FIG. 4) and 53 (FIG. 5) could be attached and connected to ground 22.In this case, the distance from nonlinear crystal 11 to ground 22 shouldbe kept to a minimum.

It has been shown that electrical discharges in nonlinear crystals 11can occur both in internal regions and regions near the surfaces. Thesenear surface discharges will be dissipated in all regions near orcovered by electrically conductive material 31 (FIG. 3), 41 (FIG. 4) or53 (FIG. 5). Those regions which are distant or not covered may still beable to damage nonlinear crystals 11. To also minimize this damage,nonlinear crystals 11 could be placed into an evacuated enclosure. Theassociated vacuum could be in the range of 10⁻⁵ to 10⁻⁶ Torr.

The foregoing description of the preferred embodiments of the inventionhave been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and obviously many modifications and variations arepossible in light of the above teaching. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical application to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

What is claimed is:
 1. A method of minimizing electrochromic andphotochromic damage in nonlinear crystals caused by a laser beamcomprising the steps of:depositing electrically conductive material ontotwo parallel surfaces of said nonlinear crystals, said two parallelsurfaces being parallel to said laser beam; and grounding saidelectrically conductive material.
 2. A method of minimizingelectrochromic and photochromic damage in nonlinear crystals caused by alaser beam comprising the steps of:depositing electrically conductivematerial onto four surfaces of said nonlinear crystals, said foursurfaces being parallel to laser beam; and grounding said electricallyconductive material.
 3. A method of minimizing electrochromic andphotochromic damage in nonlinear crystals caused by a laser beamcomprising the steps of:diffusing an electrically conductive materialinto two surfaces of said nonlinear crystals, said two surfaces beingparallel to laser beam; and grounding said electrically conductivematerial.
 4. A method of minimizing electrochromic and photochromicdamage in nonlinear crystals caused by a laser beam comprising the stepsof:diffusing an electrically conductive material into four surfaces ofsaid nonlinear crystals, said four surfaces being parallel to laserbeam; and grounding said electrically conductive material.
 5. The methodas described in claim 2 further comprising the step of depositing saidelectrically conductive material onto portions of surfaces of saidnonlinear crystal perpendicular to said laser beam, said electricallyconductive material deposited onto said perpendicular surfaces definingan opening for passage of said laser beam.
 6. The method described inclaim 4 further comprising the step of diffusing said electricallyconductive material into portions of surfaces of said nonlinear crystalperpendicular to said laser beam, said electrically conductive materialdiffused into said perpendicular surfaces of said nonlinear crystaldefining an opening for passage of said laser beam.